A scald layer about 5 μm to 15 μm thick is commonly formed on a surface of a hot-rolled steel sheet (steel strip), e.g., a hot-rolled carbon steel sheet, a high strength steel sheet, a silicon added steel sheet for an electric motor, or a stainless steel sheet.
For example, FIG. 1 illustrates a well known facility for removing the scale formed on the surface of a hot-rolled steel sheet.
That is, as illustrated in FIG. 1, the generally known typical pickling facility for descaling removes scale while a steel sheet stepwisely passes through a pickling tank 240 and a washing tank 250, after cracks are formed in the scale by using a scale breaker 230.
The pickling tank 240 is typically composed of four unit pickling tanks 242, 244, 246, and 248, and the picking tank mainly uses a high-temperature strong acid, such as hydrochloric acid or sulfuric acid, to chemically remove scale from the surface of the hot-rolled steel sheet passing therethrough.
At this time, undescribed reference numerals in FIGS. 1, 210, 220, and 260 respectively denote a pay-off reel, a welder, and a tension reel, and, although not specifically illustrated, the washing tank 250 may be divided as brushing and rinsing operations.
However, with respect to the foregoing typical scale treatment method illustrated in FIG. 1, there are various limitations, such as an increase in the length of the facility due to the use of a plurality of pickling tanks and washing tanks, obstruction of working environments due to the generation of acid vapors, the generation of environmentally hazardous waste products due to waste acid treatment, an increase in required supplementary facilities due to acid recovery and the need for acid resistant equipment, differences in descaling performances according to steels, and the occurrence of quality failures in the case of steel sheets remaining in acid solution tanks and washing tanks in the case that the production line stops.
For example, although schematically illustrated in FIG. 1, since the lengths of the actual pickling tank 240 and the washing tank 250 are about 100 meters in a method of removing scale through the foregoing typical pickling treatment, the surface qualities of hot-rolled steel sheets remaining in the pickling tank and the washing tank may be deteriorated due to an excessive acid treatment in the case that the production line stops.
Meanwhile, as an another method of removing scale for addressing the limitations of the typical method of removing scale by the medium of chemical pickling by using the foregoing acid solution illustrated in FIG. 1, a technique of removing scale from a surface of a steel sheet by using a physical method has been developed, in which metal shot balls or grit particles are in contact with the surface of the steel sheet or a slurry having metal shot balls or grit particles mixed with water is in contact with the surface of the steel sheet by using a centrifugal force.
However, the physical descaling method using shot balls or grit particles is performed in parallel with a known pickling process, in which the physical descaling method using shot balls or grit particles is mainly used as a pretreatment process for pickling to increase pickling efficiency in special steels, such as stainless steel sheets or electrical steel sheets, steels traditionally difficult to be pickled. For example, the physical descaling process using shot balls or grit particles is not applied to general carbon steels.
Meanwhile, another physical method for removing scale has been developed, in which a slurry having stainless steel (STS) beads mixed with water, instead of typical shot balls or grit particles, is in contact with a surface of a steel sheet through centrifugal force.
However, with respect to descaling methods, in which metal shot balls or grit particles are in contact with the surface of the steel sheet or a slurry having stainless steel beads mixed with water is in contact with the surface of the steel sheet through centrifugal force, since an average surface roughness (Ra) of the steel sheet after descaling is in a range of 2.3 μm to 3.5 μm due to a phenomenon of embedding in the steel sheet, the surface roughness of the descaled steel sheet may be entirely non-uniform. In addition, a unit price of the stainless steel beads may be high.
Meanwhile, other descaling techniques, such as a method of removing scale through the injection of high-pressure water or a method of injecting ice particles, have been developed, but descaling efficiencies thereof may be insignificant, and the commercialization thereof may be difficult.